Mum competed, taking on terribly hot gruelling event (8)
I believe the answer is:
marathon
'gruelling event' is the definition.
'marathon' can be an answer for 'event' (marathon is a kind of event). I'm unsure of the 'gruelling' bit.
'mum competed taking on terribly hot' is the wordplay.
'mum' becomes 'ma' (ma can mean mother).
'competed' becomes 'ran' (running is a kind of competing).
'taking on' is an insertion indicator.
'terribly' indicates anagramming the letters.
'hot' is an anagram of 'tho'.
'ran' going around 'tho' is 'rathon'.
'ma'+'rathon'='MARATHON'
Can you help me to learn more?
(Other definitions for marathon that I've seen before include "Race; battle" , "no easy task" , "Footrace of 26 miles 385 yards" , "Footrace of approx 26 miles" , "Roman hat (anag) - race" .)